Reduce cost by defaulting to basic tier of search service

[pamelafox]

Purpose

This PR makes the default deploy cheaper by using the basic tier of search service.
The main drawback of this is that you can't change tiers of existing search services, so:

1. Anyone who is updating to this code will need to explicitly set their sku to standard, via azd env set AZURE_SEARCH_SERVICE_SKU standard
2. Anyone who starts off with basic and wants to upgrade skus will have to either re-index their data ($$) or copy their data over (there are scripts to do this, but they take some effort to run)

However, we are trying to reduce default costs of our templates, so that developers have less surprises in their bills.

Does this introduce a breaking change?

When developers merge from main and run the server, azd up, or azd deploy, will this produce an error?
If you're not sure, try it out on an old environment.

[X] Yes - See note above
[ ] No

Does this require changes to learn.microsoft.com docs?

This repository is referenced by this tutorial
which includes deployment, settings and usage instructions. If text or screenshot need to change in the tutorial,
check the box below and notify the tutorial author. A Microsoft employee can do this for you if you're an external contributor.

[ ] Yes
[X] No

Type of change

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[X] Other... Please describe: $$

Code quality checklist

See CONTRIBUTING.md for more details.

N/A

I am doing test deploys on existing environments and will also do a new environment.